1. Technical Field of the Invention
The present invention relates generally to a means for absorbing the thrust generated by the impellers in each stage of a multi-stage centrifugal submersible pump, and to a means for reducing abrasive wear in such pump.
2. Prior Art
One primary means for delivering oil from a subsurface reservoir is by mechanically pumping it to the surface. One type of pump frequently used in the industry is known as a multi-stage electric submersible pump. This type of pump includes a downhole motor coupled to a centrifugal pump. The pump is comprised of a number of impellers which in turn consist of a number of vanes. As the impeller turns driven by a central shaft, the impeller vanes impart velocity to the fluid (e.g. crude oil). As the fluid is carried to the outermost portion of the impeller vane, it is transferred to the adjoining diffuser, which is stationary. Essentially, the purpose of the diffuser is to transform the fluid velocity into hydraulic head, or pressure. In turn, the diffuser guides the fluid upward into the next impeller. A diffuser and impeller comprise one "stage" of the pumping system.
Pressure gradients and momentum transferred across the impeller create a hydraulic thrust in each stage. In most operating conditions, this hydraulic thrust is in a generally downward direction and will be referred to herein as "downthrust." Excessive downthrust can have deleterious effects on a pumping system. In the longitudinal direction, downthrust can cause the impellers to contact the adjacent diffuser with force sufficient to cause damage to these components and the shaft. Hence the downthrust must be absorbed by bearings or washers either externally or located within the pump assembly.
The present invention is directed to three problems relating to submersible centrifugal pumps, which are best described with reference to the prior art. First, radial stability of the shaft is a desirable attribute; if the shaft becomes unaligned even slightly it can rub against the diffuser or impeller hubs creating friction and ultimately wear on the components. The Swatek patent (U.S. Pat. No. 5,209,577) discloses a compliant bearing system designed to achieve radial stability of the shaft. In addition, downthrust created by the upward movement of the produced fluid is also problematic because it can cause compaction of the components comprising the stages onto one another, ultimately resulting in diminished production. As stated above, several patents are directed to solving this downthrust problem, for example, the Wilson (U.S. Pat. No. 4,741,668), and the Vandevier et al. (U.S. Pat. No. 4,678,399) patents. The Wilson patent discloses bearings, rather than the washers of the present invention. The term "bearing" generally refers to a two-component system, the components slidably engaged, typically having a layer of lubricant in between them. For instance, the bearings in Wilson ("thrust bearing assembly") are comprised of, in a very simple embodiment, a rotating thrust disc and a stationary bearing surface (plus carrier).
At least one inventor has attempted to deal with the radial stability and downthrust problems in a single invention. Bearden (U.S. Pat. No. 4,741,668) and James (U.S. Pat. No. 4,781,531) disclose such inventions. Alternatively, it may be preferable to treat the radial stability and downthrust problems separately, using different components, if, for instance, a system such as that disclosed in the Swatek patent were already being utilized to solve the radial compliance problem. The present invention is not directed to the radial stability problem, but the downthrust and related problems. Hence, the instant invention would be operable in concert with the invention disclosed in the Swatek patent, which is hereby incorporated by reference into the present application.
The earliest submersible centrifugal pumps were configured to receive the downthrust at each stage; they were known as floating-pump systems. These pumps were suitable for production systems where the hydraulic thrust generated by the stage was low. For high-production and deep wells, the thrust generated was too high for these systems to operate properly. Suitable materials could not be found from which to make the washers which bore the downthrust load in each stage.
In response to this, the full-compression pump was developed, the current state of the art device in the submersible centrifugal pump industry. In this system, the entire downthrust load is borne by a single protector thrust bearing (e.g., comprising a thrust pad and a thrust runner) located at the bottom of the protector. A protector prevents the produced fluid from contaminating the clean oil in the motor. A protector bearing is very large, much larger than load-bearing washers that receive the downthrust load from only a single stage. In addition, the protector bearing cannot be positioned in contact with the produced fluid. One major drawback to the full-compression system is that, since the impellers are in a fixed position (i.e., they are "stacked" hub to hub), they cannot "reseat" against a thrust pad or washer over time, therefore, "recirculation" occurs or is exacerbated. "Recirculation" refers to the retrograde movement of liquid in the impeller, not upward to the adjacent diffuser, but downward back into the impeller entry. This does not occur--or at least not for the same reason--in the floating-type pumps because the position of the impellers can "float" or move longitudinally along the shaft. Naturally, recirculation reduces overall hydraulic efficiency hence total production (the volume of fluid leaving the wellhead), and therefore is undesirable. A "recirculation path" is cut by the gradual erosion of portions of the impeller by abrasive particles contained in the fluid (e.g., sand). In pump designs where the impeller is not fixed to the shaft, the impeller can reposition against a washer as it is worn by abrasion, thus curtailing recirculation by cutting off or at least reducing the size of its path. But in a full-compression design, the impellers' positions are fixed (longitudinally) hence as a recirculation path is enlarged due to abrasion, the impeller cannot reposition against another portion of the pump to seal the gap. In addition, the full-compression pump design requires a more difficult installation procedure, and the high shaft loads associated with it also require more costly thrust bearings in the motor protector unit. The present invention is directed towards the shortcoming of both the floating-type pumps, and the full-compression pumps. It is an advance over the art from the point of view of full-compression pumps in that, since the downthrust is preferably, though not necessarily, handled by a load-bearing washer at each stage, the impellers can move longitudinally along the shaft. Inventions directed toward the downthrust load-bearing means positioned at each stage are Sheth (U.S. Pat. No. 4,838,758) and Bearden (U.S. Pat. No. 4,741,668). Yet neither invention exploits this movement by providing in one aspect of the invention a sealing washer, or a washer, whose essential function is to effect a seal between the diffuser and impeller, as the impeller shifts and then reseats against the diffuser due to inevitable movement caused by abrasion. Therefore, the present invention is comprised of two features, though not operatively engaged to one another in the pump device, nonetheless work together toward a single purpose--the load-bearing inner thrust washers of the present invention permit the impellers longitudinal movement along the shaft, which in turn allows the impeller to reseat against the diffuser, this reseating or repositioning takes place upon the sealing washer of the present invention, which reseals the gap between the diffuser and the impeller (exterior to the impeller eye) as the impeller shifts; the washers thus reduce recirculating fluid.